Such portable measuring instruments are already known. These portable devices are particularly employed for detecting, by optical means, the heart rhythm and/or the level of oxygen in a patient's blood. They are found in various forms ranging from clamps intended to be placed on a zone of the human body (typically on the end of a finger, on the earlobe or any other extremity of the human body sufficiently irrigated by blood) to devices worn on the wrist having a similar appearance to a wristwatch.
Within the scope of an application to measurement of the heart rhythm, the illumination device is used for generating adequate illumination of a part of the organic tissue (typically the skin) and is associated with one or several photoreceptors for detecting the intensity of the luminous emission produced by the illumination device after propagation in the organic tissue. Variations in the blood flow pulsation induce a variation in the absorption of the luminous emission produced by the illumination device, the frequency of the absorption variation essentially corresponding to the frequency of the heart pulsations. Detection of the intensity of the luminous emission after propagation in the organic tissue accompanied by adequate processing of the measurement signal or signals enables to extract an indication of the heart rhythm. The illumination devices commonly used for this type of application are relatively simple and typically consist of one or several quasi-punctual light sources. They are typically LEDs (light emitting diodes) emitting within a determined wavelength range. The detection devices commonly include one or several photoreceptors formed for example of photodiodes or phototransistors.
US Patent Application No. 2002/0188210 A1 discloses for example a portable heart rhythm detection instrument intended to be worn on the inner part of the wrist, in proximity to the wrist artery. In this example, it is proposed, in particular, to arrange several photoreceptors around a single light emitting diode. Alternatively, it is further proposed to dispose several light emitting diodes around a single photoreceptor, this alternative however being undesirable because of the increase in energy consumption and the proportional increase in manufacturing and construction costs.
In all cases, the light source or sources are, as already mentioned, quasi-punctual sources whose cone or emitting surface typically only covers an area of a few mm2. This reduced emitting or illumination surface has a drawback for applications requiring illumination of an organic tissue insofar as the organic tissue has so to speak systematically localised zones where the degree of light emission absorption differs greatly from neighbouring zones. In the case of illumination of a user's skin for example, moles, hairs or other localised modifications in the epidermis can thus cause a significant variation in the light emission absorption and have a negative influence on the quality of measurement of the desired physiological quantity. Any relative movements between the measuring device and the illuminated organic tissue further add to the degradation of the measurement quality. It will be understood that these problems also arise in relation to the detection device.
It is thus desirable to illuminate the organic tissue surface more uniformly and over a larger area in order to reduce or minimise as far as possible the influence of these localised “defects” in the illuminated tissue. One solution might consist in multiplying the light sources to cover a larger area of the organic tissue. As already mentioned with reference to US document No. 2002/0188210 A1 hereinbefore, this solution is not, however essentially desirable for reasons of energy consumption and costs. It should however be noted that the increase in the number of light sources also generates connection and construction problems.
It is desirable alternatively or by way of complement, to pick up the light emission emanating from the organic tissue after propagation in the latter over a wider area in order to minimise the influence of localised defects on the signal detection quality.
It is a general object of the present invention thus to propose a portable measuring instrument for minimising the influence of localised defects in the organic tissue on the quality of illumination and/or detection of the light emission after propagation while keeping energy consumption as low as possible in order to make such a solution better suited for portable applications.
It is another object of the present invention to propose a solution whose construction remains relatively simple and whose bulkiness, particularly the thickness, remains reduced.
It is yet another object of the present invention to propose a solution which optimises as far as possible the available surface on the instrument to assure the widest possible illumination of the organic tissue and/or to assure detection of the light emission after propagation in the organic tissue over a wide area.